Field of the Invention
The present invention relates to an overheat protection circuit, and a voltage regulator equipped with the same.
Background Art
There is a case where a voltage regulator is large in power loss in an output transistor built therein and results in destruction itself due to its self-heating when a load current is large. There has therefore been provided an overheat protection circuit which stops the supply of power to a load at a predetermined temperature or higher.
FIG. 4 is a circuit diagram of a related art overheat protection circuit provided in a voltage regulator. The related art overheat protection circuit is equipped with a diode 61, current sources 62 and 64, resistors 2, 3 and 63, a comparator 65, PMOS transistors 1 and 66, a ground terminal 100, a power supply terminal 101, and an output terminal 102. Though a divided voltage Vfb at a connection point of the resistors 2 and 3 is not illustrated in the figure, but connected to an error amplifier circuit of the voltage regulator. The error amplifier circuit controls an output transistor 1 according to a reference voltage Vref and the divided voltage Vfb.
As a current flowing through the diode 61, a constant current is made to flow by the current source 62. When the constant current is made to flow through the diode 61, the temperature dependence relative to the forward bias of the diode 61 becomes about −2 mV/° C. in the case of silicon. The reference voltage Vref is generated at a connection point of the resistor 63 and the current source 64 and input to a non-inversion input terminal of the comparator 65. When the voltage on the cathode side of the diode 61 is assumed to be Vf, and the voltage of the power supply terminal 101 is assumed to be VDD, the voltage Vf is a voltage obtained by subtracting a forward bias voltage of the diode 61 from the power supply voltage VDD. The voltage Vf becomes Vf<Vref in a normal state, and hence the PMOS transistor 66 is turned off by the output of the comparator 65 to stop control on the PMOS transistor 1.
When the voltage Vf changes at the rate of about −2 mV/° C. with a rise in the temperature of an IC and becomes equal to the reference voltage Vref, the output of the comparator 65 is inverted so that the PMOS transistor 1 is turned off. Consequently, heat is not generated in the PMOS transistor 1 and hence the temperature of the IC is lowered. Then, when the voltage Vf becomes smaller than the reference voltage Vref, the output of the comparator 65 is inverted again so that the PMOS transistor 1 is turned on (refer to, for example, Patent Document 1).